ISO - Light Sensitivity

ISO adds another parameter to the Exposure equation. Till now we had established the relationship of exposure to aperture and shutter speed. From now on we will understand that Exposure is directly proportional to the ISO setting.

ISO is a measurement of the light sensitivity of the camera. Earlier we used to get films with various ISO ratings. A film with a higher ISO is more sensitive to light and on simple point-and-shoot cameras, a high ISO film would be more suitable to shooting in low light than a film with a lower ISO. In essence, that is what a high ISO will help you do - help you shoot in low light.

ISO might sound useful, but, on digital cameras, shooting at high ISO leads to an increase amount of noise in the photo. Noise is nothing but the graininess that creeps into images when you shoot at a high ISO. The noise happens due to various causes, the main reason being the over-heating of the sensing elements in the sensor of the camera. Putting it simply, to increase the light sensitivity of the sensor, more current is applied through the sensing elements. Because of this, the sensory elements are over heated and hence the resulting noise in the final images. Hence high ISO is not all that good for low light photography.

Now why would you need to shoot in low light using a high ISO when you could as well decrease the shutter speed and/or increase the aperture. For one, increasing the aperture is not an option beyond the limits of the lens. You can only increase the aperture up to the maximum value allowed by the lens. So the other option is to decrease the shutter speed to allow more light. This is possible depending on the situation. If you are trying to capture moving subjects(say a moving car) then you cannot keep a long shutter speed as you would end up capturing just a streak of color. Also, a longer shutter speed would give ghosting or blurry images if the photos are taken handheld(as opposed to a tripod). This is due to the induced 'shake' which results in longer exposures. Similarly, if you are capturing a night shot with lights in the picture, then you might end up getting streaks in the image as a result of the light moving around the screen because of camera shake.

In such cases, a compromise over the noise introduced has to be worked out and you would have to decide between decreasing the shutter speed or increase the ISO at the cost of higher noise.